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High Strength, High Toughness Microalloyed Steel Forgings Produced with Relaxed Forging Conditions and No Heat Treatment

Stein, Aaron (2020) High Strength, High Toughness Microalloyed Steel Forgings Produced with Relaxed Forging Conditions and No Heat Treatment. Master's Thesis, University of Pittsburgh. (Unpublished)

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Three steel compositionswere designed and investigated as possible materials for the forging of a wheel hub geometry using the recrystallization controlled forging (RCF) process. Titanium nitride (TiN) technology was utilized to control prior austenite grain sizes (PAGS) and vanadium precipitation to strengthen ferrite. For processing, two cooperating systems were investigated, RCF, forrefinement of the PAGS, and controlled cooling, where interrupted direct quenching (IDQ) and indirect accelerated cooling (IAC) providedmultiple possible physical property combinations. The response of the steels to thermo-mechanical processing (TMP) was investigated to determine optimal forging conditions for refinement of the PAGS. The maximum reheat temperature to avoid grain coarsening, as well as the minimum forging temperature for complete recrystallization was determined.Ti and V additionswere found toelevate both temperatures. After, IDQ and IAC cooling paths were performed on the steels to yield desired microstructure products. Microstructures comprising predominantly polygonal ferrite,bainite and martensite were attainable in the laboratory samples. Finally, the steels were tested atMeadville Forging Company (MFC), a collaborator in this project. Here, each steel was subjected to the MFC standard forging routineand varying cooling paths were used. Mechanicaltesting samples were machined and tested from the forgings. The strengths increased with cooling rate and alloying, but the fully forged, hotwater quenchedbainiticsamplesdid not perform well, having lower strengthsthan the fast air cool to room temperature (ACRT) conditions. This is interpretedto be because of
vthe low alloying, specifically the low carbon, limiting the strength of the bainite when present in large bainite phase percentages. The performance of the steels is compared with thoseof similar steelsand discussed from metallurgical standpoints. Methods of improving upon the current design of the steels are discussed. This research has demonstrated that low-carbon compositions, when subjected to proper RCFprocessing, are capable of being refined to equivalent circular prior austenite grain diameters approaching 10μm. Additionally, ultimate tensile strengths in the non-water quench to room temperature (WQRT)conditions were observed between 550MPa and 720MPa, and approaching 940MPa in the WQRT condition, with good toughness in all conditions.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Stein, Aaronaes151@pitt.eduaes1510000-0003-4749-1014
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Thesis AdvisorDeArdo, Anthonydeardo@pitt.edudeardo0000-0003-3485-4496
Committee ChairGleeson, Brianbmg36@pitt.edubmg360000-0001-5531-7337
Committee MemberSmolinski, Patrickpatsmol@pitt.edupatsmol
Committee MemberOyler, Johnoyler1@pitt.eduoyler10000-0002-2105-1654
Date: 28 January 2020
Date Type: Publication
Defense Date: 20 November 2019
Approval Date: 28 January 2020
Submission Date: 25 November 2019
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 195
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Materials Science and Engineering
Degree: MS - Master of Science
Thesis Type: Master's Thesis
Refereed: Yes
Uncontrolled Keywords: Recrystallization Controlled Forging, RCF, Titanium Nitride, TiN, Controlled Cooling
Date Deposited: 28 Jan 2020 18:03
Last Modified: 28 Jan 2020 18:03


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